Dissipation Function: Nonequilibrium Physics and Dynamical Systems

An exact response theory has recently been developed within the field of Nonequilibrium Molecular Dynamics. Its main ingredient is known as the Dissipation Function, Ω. This quantity determines nonequilbrium properties like thermodynamic potentials do with equilibrium states. In particular, Ω can be used to determine the exact response of particle systems obeying classical mechanical laws, subjected to perturbations of arbitrary size. Under certain conditions, it can also be used to express the response of a single system, in contrast to the standard response theory, which concerns ensembles of identical systems. The dimensions of Ω are those of a rate, hence Ω can be associated with the entropy production rate, provided local thermodynamic equilibrium holds. When this is not the case for a particle system, or generic dynamical systems are considered, Ω can equally be defined, and it yields formal, thermodynamic-like, relations. While such relations may have no physical content, they may still constitute interesting characterizations of the relevant dynamics. Moreover, such a formal approach turns physically relevant, because it allows a deeper analysis of Ω and of response theory than possible in case of fully fledged physical models. Here, we investigate the relation between linear and exact response, pointing out conditions for the validity of the response theory, as well as difficulties and opportunities for the physical interpretation of certain formal results

Graphitization effects induced by thermal treatments of 4H-SiC

4H-SiC is one of the most promising indirect wide-bandgap (3.3 eV) semiconductor for power devices used in the emerging area of high-voltage and high-temperature electronics as well as space and radiation harsh environments applications. The wide diffusion of devices in SiC is related to the high quality of the crystals, both for substrates and epitaxial layers. In this work, we performed thermal treatments in Argon atmosphere at temperatures below 2000°C with the aim to study the thermal stability of substrates of 4H-SiC. The wafer substrates were characterized by micro-Raman spectroscopy, Atomic Force Microscopy and Electrostatic Force Microscopy. The thermal treatments induced inhomogeneity of the wafer surface due to a graphitization process starting from 1600°C